An apparatus for cranking a vehicle engine and generating alternating current (which is hereinafter referred to as a starter-alternator) is well known. One such starter-alternator is disclosed in Japanese Pat. application No. 56-176,498 entitled "ENGINE STARTING AND CHARGING EQUIPMENT" filed Nov. 5, 1981 and laid open May 13, 1983, which comprises a flywheel with field poles formed on the outer periphery thereof, an exciting field winding disposed radially inside and radially spaced apart from the field poles of the flywheel with a slight clearance, and a stator winding assembly disposed surrounding the field poles of the flywheel for generating a revolving magnetic field. In such a starter-alternator, the flywheel with the field poles is used as a member for bearing an element of a clutch mechanism to connect or disconnect the transmission of power from an engine crankshaft of the vehicle engine and an input shaft of a transmission case. The starter-alternator thus constructed not only allows itself to be incorporated snugly between the vehicle engine and the transmission case in a power train of the vehicle but smoothly changes the transmission of engine power by controlling an amount of current flowing through the stator winding so as to accelerate or decelerate the flywheel.
A problem with such a starter-alternator is that functional activities decline due to aging. This decline of functional activities of these windings makes it hard to conduct a current because of an increased electric resistance due to heat generated in the field winding and stator winding, heat produced by friction between a clutch plate of a manual transmission case and the flywheel disposed between an engine clutch housed in a clutch housing, or heat transmitted by a fluid of a torque converter of an automatic transmission.
In particular, because of intense heat emanating from the starter winding in which a current flow is frequently alternated in the opposite directions, problems are experienced when restarting the vehicle engine by the starter-alternator when the vehicle engine is hot.